This invention relates to the mounting of a surface acoustic wave filter package to a circuit board and, more particularly, to improved grounding and effective input to output shielding of such a package.
Surface acoustic wave filters are commonly used as bandpass filters in electronic devices. Such a filter is usually packaged within an outer metal case having a substantially planar base out of which extend posts for mounting to a circuit board and for providing interconnections between the filter and traces on the circuit board. Typically, there is an input signal post insulated from the metal case, an output signal post insulated from the metal case, and one or more ground posts connected to the bases These posts extend through respective apertures in the circuit board, as is conventional, and the package is secured to the circuit board by soldering the posts to respective traces on the board. In addition, the circuit board typically has a ground plane on its upper surface for direct contact by the package base. A problem with this construction and mounting is that circuit boards usually have imperfections which result in the formation of a gap between the planar base of the filter package and the ground plane on the board. This gap provides a sneak path under and around the filter package for a portion of the input signal which is radiated from the input signal post and received by the output signal post Thus, part of the signal appears unfiltered at the output signal post, thereby lowering the efficiency of the filter. It would therefore be desirable to eliminate this sneak path.
Prior attempts to solve this problem have utilized a full body ground gasket or have soldered the entire periphery of the package to the ground plane on the board. Both of these methods are undesirable since they are difficult to implement and/or cause unacceptable damage to both the package and circuit board when repair is necessary. It would therefore be desirable to provide an effective shield and ground which does not suffer from these disadvantages.